Functional analyses of the leech swim oscillator

The oscillations that underlie swimming movements in the leech arise from a series of identified concatenated circuits within the ventral nerve cord. In the intact nerve cord, ascending and descending intersegmental interacti ons via axons within the lateral connectives function both to generate robu st oscillations throughout the cord and to establish an anterior-to-posteri or phase delay among segmental oscillators. We addressed two questions abou t this system. First, do the intrasegmental swim circuits in each ganglion function as a single oscillator or do they comprise a pair of coupled oscil lators? Second, what are the relative strengths of the ascending and descen ding intersegmental interactions between the segmental oscillators? Experim ents were carried out on semi-intact leeches (Hirudo medicinalis) and on is olated leech nerve cords in which "Z-cut" ganglia were generated by cutting one lateral connective nerve anterior and the contralateral connective ner ve posterior to the target ganglion. In these Z-cut ganglia, all rhythmic a scending intersegmental input is conveyed via one lateral connective while rhythmic descending input is conveyed via the contralateral connective. We found that rhythmic bursting recorded from the left and right sides of Z-cu t ganglia had identical cycle periods with no phase difference, despite str ong intersegmental inputs with differing periods from the two swimming ends of the preparations. We conclude that the swim circuits within individual leech ganglia act as single units. Moreover, we determined through correlat ion and Fourier spectral analyses, that the functional strengths of ascendi ng and descending intersegmental inputs to Z-cut ganglia located in the mid dle of the nerve cord are approximately equal.